The present invention relates to continuous ink jet printing and, more particularly, to an improved shutdown sequence for continuous ink jet printers.
Current ink jet printing systems consist of a fluid system supporting one or more printheads. Typical ink jet printheads operate by forcing fluid through a droplet generator which contains an array of orifices, forming droplets of ink. The printhead is fully supported by the fluid system, controlling different solenoid valves and pumps to perform necessary functions for the printhead to operate reliably. These functions include cleaning, startup, and shutdown. One particular function, shutdown, provides a means to stop operation of the printhead and fluid system over an extended period of time and allow for returned operation. Ink, or even flushing fluid, if left in the drop generator, can dry in and around the orifices, leaving behind non-volatile components in the form of solids or gels. Upon subsequent startups, the failure to remove or redissolve all of this material in and around the orifices creates disturbances in the shape or direction of the emerging jets.
In the prior art, U.S. Pat. No. 5,463,415, describes the operation of shutting down a printhead of an ink jet printing system. Shutdown typically consists of applying high vacuum to the outlet of a droplet generator with the inlet open to atmosphere through a filtered restriction. The air drawn into the drop generator through the filtered restriction and through the drop generator orifices removes the ink from the interior of the droplet generator. However, longer orifice array lengths and smaller orifices are being required in order to increase printing speeds and improve print quality.
With the changes in the droplet generator, this method of extracting the ink from the droplet generator has been found to be inadequate. In particular, it has been found that the air flow rates through the droplet generator that can be produced in this manner are insufficient to remove significant amounts of ink from the droplet generator.
The fluid system described in U.S. Pat. No. 6,273,103, overcame the air flow limitations of preceding systems by including an air pump to pump air into the inlet of the droplet generator as the system vacuum is extracting ink and air from the droplet generator outlet. This change has been found to greatly enhance ink removal from the droplet generator.
It has been found, however, that even with the increased air flow provided by existing fluid systems, small amounts of ink may still remain in sections of the droplet generator. Such ink, when allowed to dry, can result in a failure during the subsequent startup.
It would be desirable, then, to have a new shutdown procedure to solve the problem of ink and/or flush fluid drying in or around orifice holes on higher resolution printheads.
This need is met by the improved shutdown technique according to the present invention. The shutdown sequence of the present invention differs from the prior art in that the present invention proposes pulse modulation of air pressure to blow and dry the printhead filters while balancing air flow and negative pressure across the droplet generator and the orifice array.
The present invention dries and removes any type of fluid in the droplet generator and orifice array by pulsing moderately high air flow and pressure to dry the printhead final filters while keeping the droplet generator and orifice array at a negative pressure.
In accordance with one aspect of the present invention, a system and method are provided to improve reliability for shutting down printheads for a continuous ink jet printer. This system and method dries and removes ink residues and other fluids, debris and deposits in the droplet generator and orifice array by pulsing moderately high air flow and pressure to dry the printhead final filters while keeping the droplet generator and orifice array at a negative pressure.
Other objects and advantages of the present invention will be apparent from the following description and the appended claims.